-- approximately 830 last night July 21. The American flags were visible. Flying on the Brooklyn tower and the Manhattan tower of the Brooklyn Bridge. At approximately 3:10 in the morning which would be July 22. Video shows us a number of people crossing the bridge. Four or five people in a group and some other individuals. Those people will be of particular interest in this investigation. Because around 3:30 in the morning several minutes later. The light that normally illuminates the American flag. On the Brooklyn side of the bridge first -- is and then appears to go out. At 3:42. -- -- the same thing occurs on. Is visible flickering and then appearing to go out. Early this morning at approximately 530. Construction workers on the bridge noticed that the -- that normally fly in the red white and blue. Had been replaced by what appeared to be white flags. Members of the emergency service unit went to the top of the bridge to inspect and document the scene. What they found were two flags one on each tower that were approximately twenty feet by eleven -- That are actually. Appeared to be Americans. -- and stripes that had been bleached white. They also found in -- pictures indicating this to my left. What appeared to be large. Aluminum cans that had been fixed as covers. Over the lights and secured would zip ties. In total because of the ability to. Climb up to the top of the tower which is guarded by a locked gate and at the time that the emergency service people went up to inspect the scene. Those gates were in place and they were locked. For someone to compromise that -- by going around and continue the top of the tower. Have the right size. -- to put over the the light there's some indication. Some good deal of pre operational planning. Perhaps some indication that they have experience climbing. In construction or in bridge work or that they. Four looking at the dimensions we have a number. Of investigative steps we're going to go through. When we get to the questions and answers. -- happen answer questions about that. Obviously of anybody has information. -- certainly looking to hear from them about this case. In addition we are looking across. Social media FaceBook Twitter. And other indications. Or other indicators to see if there's any claim of responsibility for this that this that this time it appears. That has no particular nexus to terrorism. Or even politics. This may be somebody's art project or it may be an attempt at making some kind statement. But at this point it's not clear what that statement -- Commissioner. It was to say no matter what the motive was that is man. Analysts concerned. -- happy about the event and have -- We should know what you. Conducting its role in. So investigation into the circumstances. Again -- requesting the help of the public on this issue.

This transcript has been automatically generated and may not be 100% accurate.

Happy Birthday, Brooklyn Bridge

The national landmark's 125th year is celebrated with fireworks.

{"id":24669579,"title":"NYC Police Commissioner 'Not Particularly Happy' About White Flag Incident","duration":"3:35","description":"Commissioner William Bratton and Deputy Commissioner John Miller give an update on white flags hung on top of Brooklyn Bridge.","section":"GMA","mediaType":"Default"}